1. Field of the Invention
The invention relates to data processing systems and more particularly to a method and apparatus for interconnecting a plurality of computer workstations with I/O devices and other workstations which allows the users to share I/O resources.
2. Description of the Related Art
A Local Area Network, or LAN, is a data communications system which allows a number of independent devices to communicate with each other within a moderately-sized geographical area. The term LAN is used to describe networks in which most of the processing tasks are performed by a workstation such as a personal computer rather than by a shared resource such as a main frame computer system.
With the advent of the inexpensive personal computer workstation, LANs equipped with various kinds of desktop computers are beginning to replace centralized main frame computer installations. The economic advantage of a LAN is that it permits a number of users to share the expensive resources, such as disk storage or laser printers, that are only needed occasionally.
In a typical LAN network a desktop workstation performs processing tasks and serves as the user's interface to the network. A wiring system connects the workstations together, and a software operating system handles the execution of tasks on the network. In addition to the workstations, the LAN is usually connected to a number of devices which are shared among the workstations, such as printers and diskstorage devices. The entire system may also be connected to a larger computer to which users may occasionally need access. Personal computers are the most popular desktop workstations used with LANs.
The configuration of the various pieces of the network is referred to as the topology. In a star topology a switching controller is located at the center or hub of the network with all of the attached devices, the individual workstations, shared peripherals, and storage devices, on individual links directly connected to the central controller. In the star configuration, all of these devices communicate with each other through the central controller which receives signals and transmits them out to their appropriate destinations.
A second kind of topology is the bus topology. In this topology, wiring connects all of the devices on the LAN to a common bus with the communications signal sent from one end of the bus to the other. Each signal has an address associated with it which identifies the particular device that is to be communicated with. Each device recognizes only its address.
The third topology employs a circular bus route known as a ring. In a ring configuration, signals pass around the ring to which the devices are attached.
Both bus and ring networks are flexible in that new devices can be easily added and taken away. But because the signal is passed from end to end on the bus, the length of the network cable is limited. Star topologies have the advantage that the workstations can be placed at a considerable distance from the central controller at the center of the star. A drawback is that star topologies tend to be much slower than bus topologies because the central controller must intervene in every transmission.
In a star configuration, the signaling method is different than in bus or ring configurations. In the star configuration the processor central controller processes all of the communication signals. In a bus topology there is no central controller. Each device attempts to send signals and enter onto the bus when it needs to. If some other device trys to enter at the same time, contention occurs. To avoid interference between two competing signals, bus networks have signaling protocols that allow access to the bus by only one device at a time. The more traffic a network has, the more likely a contention will occur. Consequently, the performance of a bus network is degraded if it is overloaded with messages.
Ring bus configurations have even more complex signaling protocols. The most widely accepted method in ring networks is known as the token ring, a standard used by IBM. An electronic signal, called a token, is passed around the circuit collecting and giving out message signals to the addressed devices on the ring. There is no contention between devices for access to the bus because a device does not signal to gain access to the ring bus; it waits to be polled by the token. The advantage is that heavy traffic does not slow down the network. However, it is possible that the token can be lost or it may become garbled or disabled by failure of a device on the network to pass the token on.
The physical line which connects the components of a LAN is called the network medium. The most commonly used media are wire, cable, and fiber optics. Coaxial cable is the traditional LAN medium and is used by Ethernet.TM., the most widely recognized standard. The newest LAN transmission medium is fiber-optic cable which exhibits a superior performance over any of the other media.
The Fiber Distributed Data Interface (FDDI) is another standard. FDDI is a token-ring-implementation fiber media that provides a 100 m-bit/second data rate.
There is an increasing need for high-performance-internode communication, that is broader I/O bandwidth. The mainframe computer is being extended or replaced by department computers, workstations, and file servers. This decentralization of computers increases the amount of information that needs to be transferred between computers on a LAN. As computers get faster, they handle data at higher and higher rates. The Ethernet.TM. standard is adequate for connecting 20-30 nodes, each with a performance in the range of 1 to 5 mips. Ethernet.TM. is inadequate when the performance of these nodes ranges from 5 to 50 mips.
An I/O connectivity problem also exists that concerns I/O fanout and I/O bandwidth. The bandwidth problem was discussed above with respect to internode communication. The I/O fanout problem is related to the fact that central processing systems are getting smaller and faster. As the computing speed increases, the system is capable of handling more and more I/O. However, as the systems get smaller, it becomes harder to physically connect the I/O to the processors and memory. Even when enough I/O can be configured in the system, the I/O connectivity cost can be prohibitive. The reason is that the core system (processors and memory) must be optimized for high-speed processors and memory interconnect. The cost of each high-speed I/O connection to the core is relatively expensive. Thus, cost-effective I/O requires that the connection cost be spread over several I/O devices. On mainframe computers, the solution to the connectivity problem is solved by using a channel processor. A channel processor is a sub-processor that controls the transfer of data between several I/O devices at a time by executing channel instructions supplied by the main processor. The main processor system is connected to several of these channel processors. Several channels can share one core connection.
It is therefore an object of the present invention to provide an improved LAN that allows high performance interdevice communication and has the ability to connect a number of I/O devices to the network.